Engines are usually designed with the ability to deliver a peak output, although most engine operation is performed well below this peak value. As such, it can be beneficial to operate with some cylinders inducting air without fuel injection, and other combusting air and injected fuel, as described in U.S. Pat. No. 6,568,177.
However, the inventor herein has recognized that when using exhaust gas recirculation in such a system, one may need two EGR systems (one for each group of cylinders) so that EGR is not fed to the cylinders not carrying out combustion coupled to a catalyst such as a three way catalyst. This is because if EGR is fed to the cylinders not carrying out combustion, emissions in the EGR can exit untreated depending on the type of exhaust system, such as in a case where each group has a separate exhaust path to the atmosphere. Thus, if only one EGR valve is provided, in some cases, only one cylinder group can ever operate in the cylinder cut-out condition, unless specific exhaust gas plumbing is used.
To overcome this disadvantage, the inventors herein have recognized that it is possible to utilized variable valve timing to replace, or supplement, the EGR system. For example, in one approach, a method is used for controlling an engine having a first and second group of cylinders, the engine having a first variable valve actuator coupled to said first group and a second variable valve actuator coupled to said second group. The method comprises:
operating said first group of cylinders to combust air and injected fuel at a first air-fuel ratio, with the first actuator adjusted based on said first air-fuel ratio; and
operating said second group of cylinder to pump air with substantially no injected fuel, with the second actuator adjusted to provide different valve lift or timing in said second group than in said first group,
Thus, by using different valve timing, and/or valve lift, for cylinders carrying out combustion from cylinders without fuel injection, it is possible to provide sufficient NOx reduction and fuel economy benefits to replace, or supplement, external exhaust gas recirculation. Furthermore, since each group of cylinder has an actuator, it is possible to alternate which cylinder group is deactivated, thereby providing even wear, without requiring two EGR valve systems.
The inventors herein have further recognized other disadvantages with prior variable cam timing approaches when applied in a system that disables fuel injection to cylinders. Specifically, disadvantages can be encountered when using the same variable cam timing value for cylinders that are operating without fuel injection as for cylinders that are carrying out combustion of injected fuel. Specifically, when transitioning from operating with some cylinders combusting and other pumping air (without injected fuel) to operation with all cylinders combusting, the valve conditions which provide the best combustion in the cylinders which had already been in operation can be different from the valve conditions which provide the best restarting combustion in the cylinders which were pumping air without injected fuel.
The above disadvantages are overcome, as one example, by a method for controlling an engine having a first and second group of cylinders, the engine having a first variable valve actuator coupled to said first group and a second variable valve actuator coupled to said second group. The method comprises:
operating said first group of cylinders to combust air and injected fuel at a first air-fuel ratio, with the first actuator adjusted based on said first air-fuel ratio; and
operating said second group of cylinder to pump air with substantially no injected fuel, with the second actuator adjusted based on whether fuel injection in said second group is being re-enabled.
Thus, by providing the re-enabled cylinders with valve operation determined to provide accurate re-starting of combustion, more reliable cylinder reactivation can be achieved.
Finally, the inventor herein has recognized that in the case when one must use the same variable cam timing value for both groups of cylinders (in the case where a common actuator is used), the optimal actuator condition varies depending on whether both the groups are carrying out combustion, or whether one group is carrying out combustion and the other is inducting air with substantially no injected fuel. Further, as discussed above, the valve conditions which provide the best combustion in the cylinders which had already been in operation can be different from the valve conditions which provide the best restarting combustion in the cylinders which were pumping air without injected fuel.
The above disadvantage can be solved by a method for controlling an engine having a first and second group of cylinders, and a variable valve adjustment mechanism for adjusting cylinder valves of both said first and second group. The method comprises, in one example:
determining whether at least one of said first and second groups is operating without injected fuel and pumping at least air; and
adjusting the mechanism to adjust valve operation of both said first and second group based on said determination.
An advantage of such operation is that overall system performance can be improved, since cylinder valve operation can be selected based on more than providing efficient fuel usage and low emissions for the combusting cylinders. For example, cylinder valve operation can also be selected taking into account the deactivated cylinders, such as pumping loss of deactivated cylinders not carrying out combustion. In other words, since the first and second group are governed by the same cylinder valve actuator (in one example, they would have the same cam timing, such as for a v-8 engine where at least some of the cylinders in the two groups are in a single bank), overall operation of the engine is improved.
In this way, it is possible, in one example, to adjust the steady state position for optimal fuel economy and emissions performance from the combined cylinder groups, since a single actuator is used for both groups. Further, in the case where there is a third and fourth group having a second actuator (such as in a V-8 engine), it is possible to alternate which cylinder group is deactivated, without requiring two EGR valves. As such, numerous advantages can be achieved.